1. Field of the Invention
The present invention relates to a driver circuit for a piezoelectric actuator for vibrating a vibration body.
2. Description of the Related Art
Piezoelectric actuators typically include an electrode provided on a material having a piezoelectric effect, such as PZT ceramics, and are basically voltage driven devices. In other words, mechanical deformation occurs in response to a voltage applied to a piezoelectric actuator, and the piezoelectric actuator typically must be resonantly driven. Resonant driving is a driving scheme in which a piezoelectric actuator or a structure coupled therewith, hereinafter, referred to as “piezoelectric device,” causes a resonance phenomenon at a specific frequency determined by its mechanical shape and dimensions, thereby obtaining increased deformation which cannot be obtained by normal voltage application.
In order to perform resonant drive, it is only necessary to apply an alternating voltage at a resonant frequency of a piezoelectric device. For example, it is only necessary to connect an oscillator circuit, which generates an alternating voltage at the resonant frequency, to a piezoelectric device via a power amplifier.
However, individual differences between resonant frequencies of piezoelectric devices occur due to manufacturing variations of piezoelectric devices and inaccuracies in the mounting location of piezoelectric actuators on vibration bodies. Thus, it is difficult to resonantly drive a piezoelectric device merely by applying an alternating signal having a fixed frequency determined previously for the piezoelectric device. In addition, adjusting the frequency of an alternating voltage applied to an individual piezoelectric device has been considered. However, the resonant frequency of a piezoelectric device greatly changes with temperature changes, and thus, it is difficult to stably resonantly drive a piezoelectric device even by with such measure.
Therefore, in the related art, a circuit has been proposed which operates to automatically determine the resonant frequency of a piezoelectric device and to generate an alternating signal at the frequency and performs resonant drive with self-excited vibration. As one example, an electrode and a terminal arranged to detect a deformation amount are provided in a piezoelectric actuator to define a three-electrode piezoelectric actuator, and a driver circuit is arranged such that a drive signal is subjected to positive feedback to the piezoelectric actuator by a signal from the terminal arranged to detect a deformation amount. In other words, this is a method in which the piezoelectric actuator is controlled and driven such that its deformation amount is maximized.
However, a method of manufacturing such a three-electrode piezoelectric actuator is complicated and the cost is high. Further, in a piezoelectric actuator having a large amplitude of vibration, a large amount of distortion occurs between a drive portion which deforms to a large extent and a portion at which an electrode arranged to detect a deformation amount, which does not autonomously deform, is provided. Thus, the piezoelectric actuator is likely to be damaged.
When a two-electrode piezoelectric actuator is used which does not include the electrode arranged to detect a deformation amount, a circuit configuration can be used in which the piezoelectric actuator is incorporated into a resonance system of a driver circuit, such that the frequency of an alternating voltage applied to the piezoelectric actuator is controlled to match the actual resonant frequency of the piezoelectric actuator.
A known circuit which performs resonant drive with self-excited vibration is disclosed in the Magazine “Fuel Cell”, written by Kamiya Gaku, Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel Cell Development Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009, P 148-151, FIG. 2. FIG. 1 shows a basic configuration of a driver circuit for a piezoelectric actuator, which is shown the Magazine “Fuel Cell”, written by Kamiya Gaku, Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel Cell Development Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009, P148-151, FIG. 2. A resistor R arranged to detect current is inserted in a current path for a piezoelectric actuator “a”. A voltage signal proportional to a current flowing in the piezoelectric actuator “a” is obtained by the resistor R, and driving at a frequency at which the voltage-current phase difference of the piezoelectric actuator “a” is substantially 0° is achieved by an operational amplifier OP to which positive feedback of the voltage signal is provided.
The piezoelectric actuator driver circuit disclosed in the Magazine “Fuel Cell”, written by Kamiya Gaku, Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel Cell Development Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009, P148-151, FIG. 2, drives the piezoelectric actuator using its self vibration, and thus, the piezoelectric actuator can always be driven at a resonant frequency so as to correspond to variations of the resonant frequency. However, a piezoelectric device includes a plurality of higher-order resonant modes, in addition to a fundamental resonant mode. These resonant modes are provided by vibration generated due to the shape and the size of the vibration body and the vibration of the piezoelectric actuator.
In the circuit shown in FIG. 1, positive feedback is provided at a frequency at which the impedance Z of the piezoelectric actuator “a” shows resistivity, i.e., at a frequency at which the reactance is approximately 0, but positive feedback is provided even at a higher-order resonant frequency other than the fundamental resonant frequency. Thus, higher-order resonant oscillation is likely to occur. In the higher-order resonant oscillation, an oscillating portion cannot be oscillated at a predetermined amplitude, and thus predetermined functions of a piezoelectric device are not achieved. In addition, highly audible noise occurs due to harmonics.